The present invention relates to a signal processing circuit for an ultrasonic diagnostic apparatus.
An ultrasonic diagnostic apparatus is so constructed that a living body is irradiated with an ultrasonic beam irradiated from an ultrasonic probe, an echo signal reflected from the living body is received and processed, and a sector image of the living body obtained is displayed on a display. The ultrasonic echo signal has a great level difference such as 60 dB. For example, if the small-amplitude signal component has an amplitude of level 1, the large-amplitude signal component has an amplitude of level 1,000. These signal components have an exponential distribution in amplitude. Furthermore, when the ultrasonic beam propagates within the living body, it is attenuated by the difference in the acoustic impedance which is, in turn, caused by the density of the body tissue, and the ratio of the signal components of the ultrasonic echo signal becomes as great as 120 dB. The attenuation characteristics also have an exponential distribution. Such an ultrasonic echo signal is amplified by a logarithmic amplifier, processed by a signal processing circuit, supplied to a monitor display and displayed on the monitor display as a sector image. During this process, the signal processing circuit performs gain control and dynamic range control of the signal to control the gradation and dynamic range. In such control operations, setting values significantly change according to the part of the body under examination or from one patient to another. Therefore, the setting values must be adjusted for every control operation. The brightness signal, i.e., the echo signal for one transmission and reception operation includes the signal component of large amplitude and the signal component of small amplitude. For this reason, even if the operator has good knowledge of the principle of the dynamic range control, an excellent sector image or a sector image of required quality may not be obtained. This is disadvantageous for simplicity of operation, speed of examination, and reliability of examination results. In order to compensate for these defects, an automatic gain control (AGC) has been adopted. However, this does not sufficiently eliminate these defects. In addition to this, the AGC has an effect of emphasizing the contour. Therefore, when the effect of the AGC is too intense, the contour is emphasized too much, degrading the image quality. Furthermore, the AGC cancels the signal component of small amplitude mixed with the signal component of large amplitude. Therefore, when the effect of the AGC is too intense, the dynamic range is narrowed.